(a) Technical Field of the Invention
The present invention generally relates to light guide plates of TFT-LCDs, and more particularly to light guide plates that can uniformly scatter lights from point light sources by widening the lights' incident angles.
(b) Description of the Prior Art
A typical planar light source found in a TFT-LCD, as shown in FIG. 1, mainly includes a light guide plate 11, a reflection plate 12, a number of diffusion films 13 and prism sheets 14, and lamps 15. As one of the most important components of the planar light source 1, the light guide plate 11 receives, transmits, and redirects lights emitted from the lamps 15.
As shown in FIG. 2, the light guide plate 11 is usually of a rectangular shape, having a light exiting side 111, a reflection side 112 opposite to the light exiting side 111, and at least a light entering side 113 at the flank. The lights emitted from the lamps 15 enter into the light guide plate 11 via the light entering side 113, of which a portion directly leaves the light guide plate 11 via the light exiting side 111, and the other portion lands on the reflection side 112. The reflection side 112, therefore, is configured with numerous dots 1121 on the surface and the dots 1121 have increasingly higher distribution density or larger surface area as they are located farther away from the lamps 15, so as to reflect light as much as possible to the fight exiting side 111.
Usually, cold cathode fluorescent lamps (CCFLs) or light-emitting diodes (LEDs) are used as the lamps 15 for the planar light source 1. The application of CCFLs is simpler, as they are linear light sources. On the other hand, the application of LEDs is more complicated as they are point light sources. As shown in FIG. 3, the lights emitted from the LEDs 16 would have an incident angle when entering the light guide plate 111 via the light entering side 113. There are, therefore, bright zones 114 and dark zones 115 formed inside the light guide plate 11, depending on whether they are within the coverage of the incident light. Although adding more LEDs 16 could eliminate dark zones 115, this is not a satisfactory approach as the additional LEDs 16 increase material cost, power consumption, assembly complexity, and the possibility of future malfunctioning.
Another approach has been proposed for eliminating dark zones. As shown in FIG. 4, the light guide plate 2 has a fight exiting side 21, a reflection side 22 opposite to the light exiting side 21, and at least a light entering side 23 at the flank. The fight entering side 23 is configured to have a plurality of saw-toothed diffusing entities 231. The lights emitted from the LEDs 25, when passing through the slant surfaces 2311 of the diffusing entities 231, are scattered to have larger incident angles. The dark zones 24 inside the light guide plate 2 are thereby reduced. However the approach has its own problem. As shown in FIG. 5, when lights emitted form the LEDs 25 passes thought the diffusing entities 231, they are refracted by the two slant surfaces 2311 of each diffusing entity 231 and some of them would have aligned traveling direction. These aligned refracted lights would add to each other to form brighter lights 26, resulting in non-uniform lighting of the light guide plate 2. This phenomenon would limit the subsequent application of the light guide plate 2 and is not consistent with the uniform lighting requirement of the light guide plate 2.